Low compression natural gas engine piston bowl for improved combustion stability

ABSTRACT

A piston may have an annular body including a crown portion defining a longitudinal axis, a radial direction perpendicular to the longitudinal axis, a plane containing the longitudinal axis and the radial direction, and a contoured combustion bowl. In the plane containing the longitudinal axis and the radial direction, the crown portion includes a radially outer squish surface, and a swirl pocket having a reentrant surface that extends axially downwardly and radially outwardly from the squish surface defining a tangent that forms a reentrant angle with the squish surface that ranges from 53.0 degrees to 57.0 degrees.

TECHNICAL FIELD

The present disclosure relates generally to pistons that are used ininternal combustion engines having a contoured piston bowl geometry witha reentrant surface. More specifically, the present disclosure relatesto a piston having a contoured piston bowl geometry with a reentrantsurface that may improve combustion stability in a low compressionnatural gas engine.

BACKGROUND

Internal combustion engines are routinely used in various industries topower machines and equipment. Examples of industries using such machinesand equipment include marine, earth moving, construction, mining,locomotive and agriculture industries, etc. In certain markets andmarket segments, natural gas engines require a low compression ratio anda good combustion stability in order to meet environmental standardswhile also being economical to run.

More specifically, it has been traditionally found that a tradeoff orcompromise exists between providing a natural gas engine with a highpower output while also having a high combustion efficiencysimultaneously. In some cases, the limits for emissions of unburnedhydrocarbons are undesirably approached, risking exceeding these limits.

India Publ. Pat. Application No. 201621045122 A (the ′122 patentapplication) discloses a piston with a recessed combustion chamber onthe piston head for improving the speed of combustion. The piston mayfurther comprise the recessed combustion chamber faced towards acylinder head with an injector. The injector is a fuel injector forintroduction of fuel into the combustion chamber, where the mixture offuel and air is allowed to burn. The inner structural design of thecombustion chamber may be formed into a circular shape of a rotatingbody with an axis in the direction of the translatory movement of thepiston. At the piston head, an opening is formed by configuring a seriesof interconnected lobes in the shape of a flower. The complete assemblyfurther increases the speed of combustion enabled by better mixing ofair and fuel which results in lowering the NOx emission and Particulatematter emission.

As can be seen, the ′122 patent application claims to improve combustionstability, but does claim to increase both combustion stability, andpower output simultaneously.

SUMMARY OF THE DISCLOSURE

A piston configured to reciprocate in the bore of an engine according toan embodiment of the present disclosure is provided The piston maycomprise an annular body including a crown portion defining alongitudinal axis, a radial direction perpendicular to the longitudinalaxis, a plane containing the longitudinal axis and the radial direction,and a contoured combustion bowl. In the plane containing thelongitudinal axis and the radial direction, the crown portion mayinclude a top squish surface, and a swirl pocket having a reentrantsurface that extends axially downwardly and radially outwardly from thetop squish surface defining a tangent that forms a reentrant angle withthe top squish surface that may range from 53.0 degrees to 57.0 degrees.

A piston configured to reciprocate in the bore of an engine according toanother embodiment of the present disclosure is provided. The piston maycomprise an annular body including a crown portion defining alongitudinal axis, a radial direction perpendicular to the longitudinalaxis, a plane containing the longitudinal axis and the radial direction,and a contoured combustion bow. In the plane containing the longitudinalaxis and the radial direction, the contoured combustion bowl may beradially surrounded by an annular cooling gallery that defines a maximumannular radial width, while the contoured combustion bowl may have aswirl pocket including a cylindrical surface defining a minimum diameterof the swirl pocket radially proximate to the maximum annular radialwidth of the cooling gallery. A ratio of the minimum diameter to themaximum annular radial width may range from 6.3 to 7.7.

A piston configured to reciprocate in the bore of an engine according toyet another embodiment of the present disclosure is provided. The pistonmay comprise an annular body including a crown portion defining alongitudinal axis, a radial direction perpendicular to the longitudinalaxis, a plane containing the longitudinal axis and the radial direction,and a contoured combustion bowl. In the plane containing thelongitudinal axis and the radial direction, the contoured combustionbowl may define a maximum axial depth measured from a planar squishsurface to a bottom concave arcuate surface defining a bottom extremityof a swirl pocket, and the swirl pocket may include a concave arcuateside surface that is spaced axially away from the squish surface, andthe bottom concave arcuate surface. The concave arcuate side surface maydefine an axial height, and a ratio of the maximum axial depth of thecontoured combustion bowl to the axial height may range from 6.9 to 8.4.

A combustion chamber of an engine comprising a bore of the engine with abore diameter, a longitudinal axis, a radial direction perpendicular tothe longitudinal axis, and a piston configured to reciprocate in thebore of the engine according to an embodiment of the present disclosureis provided. The piston may reciprocate from a top dead center positionto a bottom dead position defining a compression height therebetween.The piston may also include an annular body including a crown portionand defining a plane containing the longitudinal axis and the radialdirection, and a contoured combustion bowl. In the plane containing thelongitudinal axis and the radial direction, the contoured combustionbowl may define a maximum axial depth measured from a planar squishsurface to a bottom concave arcuate surface defining a bottom extremityof a swirl pocket. A ratio of the compression height to the maximumaxial depth may range from 5.0 to 15.0.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an internal combustion engine that mayemploy piston(s) configured according to various embodiments of thepresent disclosure.

FIG. 2 is a sectioned side view of the internal combustion engine ofFIG. 1 , showing generically a combustion chamber with a piston that maybe configured according to an embodiment of the present disclosure thatis disposed in a cylinder bore for reciprocating movement therein.

FIG. 3 is a sectional view of a piston that may be disposed in thecombustion chamber of FIG. 2 .

DETAILED DESCRIPTION

Reference will now be made in detail to embodiments of the disclosure,examples of which are illustrated in the accompanying drawings. Whereverpossible, the same reference numbers will be used throughout thedrawings to refer to the same or like parts. In some cases, a referencenumber will be indicated in this specification and the drawings willshow the reference number followed by a letter for example, 100 a, 100 bor a prime indicator such as 100′, 100″etc. It is to be understood thatthe use of letters or primes immediately after a reference numberindicates that these features are similarly shaped and have similarfunction as is often the case when geometry is mirrored about a plane ofsymmetry. For ease of explanation in this specification, letters orprimes will often not be included herein but may be shown in thedrawings to indicate duplications of features discussed within thiswritten specification.

Various embodiments of a piston with a new bowl is disclosed that maymeet engine performance goals and fit into a new piston architecture.The new bowl may have a maximized volume due to its depth and diameter,providing for a low geometric compression ratio. At the same time, arelatively high squish velocity to maximize total kinetic energy in thein-cylinder gas mixture has been maintained through utilization of areentrant feature at the bowl opening. Also, a proper ratio ofcombustion volume in the bowl versus cooling volume in a cooling gallerymay be provided. These combination of features may provide reducedpiston temperatures, reduced unburned hydrocarbons, and improvedcombustion efficiency when the engine is operated at is its rated load.

For example, an internal combustion engine 100 is shown in FIG. 1 thatmay employ various embodiments of the piston constructed according tothe principles set forth herein. The engine 100 may include an engineblock 102 in which the piston (not shown) reciprocates, and a cylinderhead 104 that may contain various engine components for the introductionof fluids into the bore/combustion chamber located in the engine block102.

Turning to FIG. 2 , a portion of the engine 100 is shown sectioned,revealing the combustion chamber 106 that may have a generallycylindrical shape that is defined within a cylinder bore 108 formedwithin the crankcase or engine block 102 of the engine 100. Thecombustion chamber 106 is further defined at one end by a flame decksurface 110 of the cylinder head 104, and at another end by a crownportion 202 of a piston 200 that is reciprocally disposed within thebore 108, and is connected to a connecting rod 124 at its connecting rodattachment portion 204, which in turn is connected to a crank shaft (notshown).

During operation of the engine 100, air is admitted into the combustionchamber 106 an air inlet passage 115 when one or more intake valves 117(one shown) are open during an intake stroke. In a known configuration,high pressure fuel is permitted to flow into a pre-chamber to mix withthe intake air to create a pre-determined fuel/air mixture. A spark iscreated by a spark plug 114 to ignite the mixture that results inburning and expanding gases 118. Following combustion, exhaust gas isexpelled from the combustion chamber through an exhaust conduit 120 whenone or more exhaust valves 122 (one shown) is/are open during an exhauststroke.

While a gas engine has been just described in detail, it is to beunderstood that the embodiments described herein may also be used indiesel and/or compression ignition engines.

The uniformity and extent of fuel/air mixing in the combustion cylinderis relevant to the combustion efficiency as well as to the amount andtype of combustion byproducts that are formed. For example, fuel-richmixtures, which may be locally present within the combustion chamber 106during a combustion event due to insufficient mixing, may lead to highernitrogen oxides and unburned carbon emissions and lower combustionefficiency.

In particular embodiments of the present disclosure as understoodlooking at FIGS. 2 and 3 together, the combustion chamber 106 defines abore 108 of the engine with a longitudinal axis L108, and a radialdirection R108 perpendicular to the longitudinal axis. A piston 200 isshown in the bore that is configured to reciprocate in the bore of theengine from a top dead center position to a bottom dead position 126 asnearly shown in FIG. 3 , defining a compression height 128 between thesepositions.

As best seen in FIG. 3 , the piston 200 may have an annular bodyincluding a crown portion 202 that defines a plane containing thelongitudinal axis L108, and the radial direction R108 when installed. Itis to be understood that the annular body of the piston may itselfdefine similar axes, and directions when not in the bore of the engine,but would be coincident or nearly coincident with those of the bore wheninstalled in the bore. Also, a skirt 204 is shown that may be a fullskirt in some embodiments of the present disclosure. This may not be thecase in other embodiments of the present disclosure. The skirt and thecrown portion may be unitary, integral, etc.

The piston also has a contoured combustion bowl 206, and in thesectioned plane of FIG. 3 , which contains the longitudinal axis L108and the radial direction R108, defines a maximum axial depth D206measured from a planar squish surface 208 (or a plane containing thissurface) to a bottom concave arcuate surface 210 defining a bottomextremity 212 of a swirl pocket 214 (so called since it promotes mixingand atomization of the fuel in the air to help improve combustionefficiency). A ratio of the compression height 128 of the piston to themaximum axial depth D206 may range from 2.05 to 2.475 (e.g., about 2.26)in some embodiments of the present disclosure. Such a range may beconsidered to provide a low geometric compression ratio. In such anembodiment, a ratio of a maximum diameter D206 of the combustion bowldefined by the concave arcuate side surface to a minimum diameter of thecombustion chamber defined by a cylindrical surface (see D220) may rangefrom 1.02 to 1.24.

Also, the contoured combustion bowl 206 may include a reentrant surface216 extending from the planar squish surface 208 at a reentrant angleA216 in the sectioned plane of FIG. 3 , and a concave arcuate sidesurface 218 that extends axially downwardly from the reentrant surface216 defining an axial height H218 of the concave arcuate side surface218. In some embodiments the reentrant angle ranges from 53.0 degrees to57.0 degrees, and a ratio of the compression height 128 to the axialheight H218 ranges from 5.0 to 15.0, or more specifically 10.0 to 12.0(may be approximately 11.0). It should be noted that the compressionheight is most accurately portrayed in FIG. 2 , as what is shown in FIG.3 is an approximation of the compression height as it generallyrepresents the amount of movement of the piston.

More specifically, the concave arcuate side surface 218 of the swirlpocket 214 may be spaced axially away from the planar squish surface 218(i.e., other surfaces are interposed such as the reentrant surface 216,etc.), as well as the bottom concave arcuate surface 210. For example,the swirl pocket 214 may include a cylindrical surface 220 (i.e. hasless than 7.0 degrees of a draft angle) that defines a minimum diameterD220 of the swirl pocket 214 that ranges from 111.0 millimeters to 114.0millimeters in some embodiments of the present disclosure. Also, a ratioof the maximum axial depth D206 of the contoured combustion bowl 206 tothe axial height H218 of the concave arcuate side surface 218 may rangefrom 6.9 to 8.4 (may be about 7.6) in some embodiments of the presentdisclosure.

Specific geometric values may include the following. A small radius 222may connect the planar squish surface 208 to the reentrant surface 216in some embodiments of the present disclosure (e.g., may have a value of0 to 0.2 mm (or about 0.1 mm in the sectioned plane of FIG. 3 )). Inaddition, the planar squish surface 208 (with the small radius 222 ifpresent) may define an entry diameter D208 that ranges from 109.0millimeters to 113.0 millimeters in some embodiments of the presentdisclosure. Similarly, the concave arcuate side surface 218 defines aminimum arcuate surface diameter D218 that ranges from 115.0 millimetersto 118.0 millimeters, while the maximum axial depth D206 of thecontoured combustion bowl 206 ranges from 41.0 millimeters to 44.0millimeters. Other dimensional ranges are possible in other embodimentsof the present disclosure such as when the design is scaled up or down,etc.

Still referring to the sectioned plane of FIG. 3 , the crown portion 202may include a top squish surface (e.g., may take the form of the planarsquish surface 208 as alluded to earlier herein), and a reentrantsurface 216 that extends axially downwardly and radially outwardly fromthe top squish surface (or planar squish surface 208) that defines atangent T216 that forms a reentrant angle A216 with the top squishsurface that ranges from 53.0 degrees to 57.0 degrees in someembodiments. The reentrant surface may take various shapes includingarcuate, or conical as shown in FIG. 3 . If so, the tangent T216, andthe reentrant surface 216 may look or act like one in the same.

As also alluded to earlier herein, the swirl pocket 214 further includesa concave arcuate surface (e.g., concave arcuate side surface 218)extending from the reentrant surface 216, defining a radius of curvatureROC218 that ranges from 8.5 millimeters to 10.5 millimeters in thesectioned plane of FIG. 3 in some embodiments of the present disclosure.If so, the center C218 of this radius of curvature may be disposed anaxial distance AC218 from the top squish surface (or a plane containingthe planar squish surface 208) ranging from 9.5 millimeters to 10.0millimeters in some embodiments of the present disclosure. Also, theconcave arcuate surface may be an exact radius, but not necessarily so.As used herein, the term “arcuate” means any surface that is not conicalor planar, and may include a radius, radii, an ellipse, a spline, apolynomial, etc.

Furthermore, a converging surface 224 may extend radially inwardly fromthe concave arcuate surface toward the longitudinal axis L108, defininga lower tangent T224 that forms an acute angle A224 with thelongitudinal axis L108 in the sectioned plane of FIG. 3 that ranges from18.0 degrees to 22.0 degrees in some embodiments of the presentdisclosure. As with the reentrance surface, the converging surface 224may also be conical, but not necessarily so.

As mentioned previously herein, the swirl pocket 214 may have acylindrical surface 220 extending axially downwardly from the convergingsurface 224, as well as a concave bottom extremity defining surface(e.g., see bottom concave arcuate surface 210 defining a bottomextremity 212) extending from the cylindrical surface 220. A convexarcuate surface 226 may extend upwardly from the concave bottomextremity defining surface to a flat plateau surface 228 (i.e., thissurface may be flat within 0.5 mm or less) that is perpendicular to thelongitudinal axis L108.

With continued reference to FIG. 3 , a radius 230 or other arcuatesurface may connect the converging surface 224 to the cylindricalsurface 220 having a radius of curvature ROC230 ranging from 13.0millimeters to 17.0 millimeters in some embodiments of the presentdisclosure. Plus, the concave bottom extremity defining surface (e.g.,may take the form of bottom concave arcuate surface 210) may have aradius of curvature ROC210 that ranges from 8.0 millimeters to 12.0millimeters, while the convex arcuate surface 226 may have a radius ofcurvature ROC226 that ranges from 50.0 millimeters to 54.0 millimetersin some embodiments of the present disclosure.

While the previous embodiments of the piston have been directed to thosethat maximize power output without sacrificing combustion efficiency orstability, the next embodiment is more concerned with pistontemperatures so that they are not too high, etc.

For example, in the sectioned plane of FIG. 3 , the contoured combustionbowl 206 may be radially surrounded by an annular cooling gallery 232that defines a maximum annular radial width W232. Moreover, thecontoured combustion bowl 206 may have a swirl pocket including acylindrical surface 220 defining a minimum diameter D220 of the swirlpocket 214 that is disposed radially proximate to the maximum annularradial width W232 of the cooling gallery. In some embodiments, a ratioof the minimum diameter to the maximum annular radial width W232 rangesfrom 6.3 to 7.7.

More particularly, the annular cooling gallery 232 may define a radiallyinner cylindrical surface 234, and a radially outer cylindrical surface236 that defines the maximum annular radial width W232 of the annularcooling gallery 232. Also, the swirl pocket 214 may further include aconical diverging surface 238 extending radially outwardly, and axiallyupwardly from the cylindrical surface 220 of the swirl pocket 214,whereas the annular cooling gallery 232 may further include a conicalsurface 240 that is radially proximate to the conical diverging surface238, and that is parallel to the conical diverging surface 238. Hence,the local wall thickness of the piston between these features ismaintained relatively constant.

Still referring to the sectioned plane of FIG. 3 , the swirl pocket 214may further have a concave arcuate surface (e.g. concave arcuate sidesurface 218) extending upwardly from the conical diverging surface 238to a reentrant surface 216 that extends to a planar squish surface 208.

At the bottom of the swirl pocket 214, an undulating surface 242 mayextend axially downwardly and radially inwardly from the cylindricalsurface 220 of the swirl pocket to a bottom extremity 212 of the swirlpocket, and then radially inwardly and axially upwardly to a flatplateau surface 228 (i.e., within 0.5 mm of being flat) and spaced awayfrom the squish surface an axial depth of about 37.0 millimeters in someembodiments of the present disclosure. In other embodiments, surface 228may be arcuate, etc.

The configuration, ratios and dimensional ranges of any of the featuresof any of the embodiments discussed herein may be altered to bedifferent than what has been explicitly discussed or shown depending onthe application.

The piston may be fabricated from steel (e.g., tool steel, stainlesssteel, etc.), cast aluminum alloy, forged aluminum alloy or othersuitable material that is durable, corrosion resistant, etc. Thegeometry of the crown portion may be formed during the casting orforging process and then may be rough machined and/or finish machined ifnecessary. Suitable machining processes may include milling, turning,electrical discharge machining, etc.

Since a turning process is used to create some or all of the finishedgeometry of the piston, it can be readily understood by one skilled inthe art that most, almost all, or all of the finished geometry of thesecomponents may not vary, or may not vary significantly, along thecircumferential direction about the longitudinal axis.

INDUSTRIAL APPLICABILITY

In practice, a piston, a crown portion of a piston, a combustionchamber, and/or an engine assembly using any of these componentsaccording to any embodiment described herein may be provided, sold,manufactured, and bought etc. as needed or desired in an aftermarket orOEM (original equipment manufacturer) context. For example, a crownportion or a piston may be used to retrofit an existing engine alreadyin the field or may be sold with an engine or a piece of equipment usingthat engine at the first point of sale of the piece of equipment.

The inventors have found that the selected bowl geometry facilitates abalance between low compression ratio, high squish velocity, andeffective cooling of the bowl rim and top land. Other designs may beable to achieve the same low compression ratio, but would likelysacrifice the desired squish velocity or the desired amount of cooling.

Put another way, various embodiments of the present disclosure break thetradeoff between increasing power output using a low compression ratio,and improving combustion efficiency simultaneously.

It will be appreciated that the foregoing description provides examplesof the disclosed assembly and technique. However, it is contemplatedthat other implementations of the disclosure may differ in detail fromthe foregoing examples. All references to the disclosure or examplesthereof are intended to reference the particular example being discussedat that point and are not intended to imply any limitation as to thescope of the disclosure more generally. All language of distinction anddisparagement with respect to certain features is intended to indicate alack of preference for those features, but not to exclude such from thescope of the disclosure entirely unless otherwise indicated.

Recitation of ranges of values herein are merely intended to serve as ashorthand method of referring individually to each separate valuefalling within the range, unless otherwise indicated herein, and eachseparate value is incorporated into the specification as if it wereindividually recited herein.

As used herein, the articles “a” and “an” are intended to include one ormore items, and may be used interchangeably with “one or more.” Whereonly one item is intended, the term “one” or similar language is used.Also, as used herein, the terms “has”, “have”, “having”, “with” or thelike are intended to be open-ended terms. Further, the phrase “based on”is intended to mean “based, at least in part, on” unless explicitlystated otherwise.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the embodiments of theapparatus and methods of assembly as discussed herein without departingfrom the scope or spirit of the invention(s). Other embodiments of thisdisclosure will be apparent to those skilled in the art fromconsideration of the specification and practice of the variousembodiments disclosed herein. For example, some of the equipment may beconstructed and function differently than what has been described hereinand certain steps of any method may be omitted, performed in an orderthat is different than what has been specifically mentioned or in somecases performed simultaneously or in sub-steps. Furthermore, variationsor modifications to certain aspects or features of various embodimentsmay be made to create further embodiments and features and aspects ofvarious embodiments may be added to or substituted for other features oraspects of other embodiments in order to provide still furtherembodiments.

Accordingly, this disclosure includes all modifications and equivalentsof the subject matter recited in the claims appended hereto as permittedby applicable law. Moreover, any combination of the above-describedelements in all possible variations thereof is encompassed by thedisclosure unless otherwise indicated herein or otherwise clearlycontradicted by context.

What is claimed is:
 1. A piston configured to reciprocate in the bore ofan engine, the piston comprising: an annular body including a crownportion defining a longitudinal axis, a radial direction perpendicularto the longitudinal axis, a plane containing the longitudinal axis andthe radial direction, and a contoured combustion bowl; wherein in theplane containing the longitudinal axis and the radial direction thecrown portion includes a top squish surface, and a swirl pocket having areentrant surface that extends axially downwardly and radially outwardlyfrom the top squish surface defining a tangent that forms a reentrantangle with the top squish surface that ranges from 53.0 degrees to 57.0degrees; the swirl pocket further including a concave arcuate surfaceextending from the reentrant surface, a bottom concave arcuate surfacedefining a bottom extremity of the swirl pocket, and a convex arcuatesurface extending between the concave arcuate surface and the bottomconcave arcuate surface.
 2. The piston of claim 1, wherein the reentrantsurface is conical.
 3. The piston of claim 1, wherein the concavearcuate surface defining a radius of curvature that ranges from 8.5millimeters to 10.5 millimeters in the plane containing the longitudinalaxis and the radial direction.
 4. The piston of claim 3, wherein theswirl pocket further comprises a converging surface that extendsradially inwardly from the concave arcuate surface toward thelongitudinal axis, defining a lower tangent that forms an acute anglewith the longitudinal axis in the plane containing the longitudinal axisand the radial direction that ranges from 18.0 degrees to 22.0 degrees.5. The piston of claim 4, wherein the converging surface is conical andthe concave arcuate surface is an exact radius.
 6. The piston of claim5, wherein the swirl pocket further comprises a cylindrical surfaceextending axially downwardly from the converging surface, a concavebottom extremity defining surface extending from the cylindricalsurface, a convex arcuate surface extending upwardly from the concavebottom extremity defining surface to a flat plateau surface that isperpendicular to the longitudinal axis.
 7. The piston of claim 6,wherein the swirl pocket further includes a radius connecting theconverging surface to the cylindrical surface having a radius ofcurvature ranging from 13.0 millimeters to 17.0 millimeters, the concavebottom extremity defining surface has a radius of curvature that rangesfrom 8.0 millimeters to 12.0 millimeters, and the convex arcuate surfacehas a radius of curvature that ranges from 50.0 millimeters to 54.0millimeters.
 8. A piston configured to reciprocate in the bore of anengine, the piston comprising: an annular body including a crown portiondefining a longitudinal axis, a radial direction perpendicular to thelongitudinal axis, a plane containing the longitudinal axis and theradial direction, and a contoured combustion bowl; wherein in the planecontaining the longitudinal axis and the radial direction the crownportion includes a top squish surface, and a swirl pocket having areentrant surface that extends axially downwardly and radially outwardlyfrom the top squish surface defining a tangent that forms a reentrantangle with the top squish surface that ranges from 53.0 degrees to 57.0degrees; the swirl pocket further including: a concave arcuate surfaceextending from the reentrant surface, defining a radius of curvaturethat ranges from 8.5 millimeters to 10.5 millimeters in the planecontaining the longitudinal axis and the radial direction; and aconverging surface that extends radially inwardly from the concavearcuate surface toward the longitudinal axis, defining a lower tangentthat forms an acute angle with the longitudinal axis in the planecontaining the longitudinal axis and the radial direction that rangesfrom 18.0 degrees to 22.0 degrees.
 9. The piston of claim 8 wherein theswirl pocket further includes a cylindrical surface extending axiallydownwardly from the converging surface.
 10. The piston of claim 9 thewherein the swirl pocket further includes a radius connecting theconverging surface to the cylindrical surface.
 11. A piston configuredto reciprocate in the bore of an engine, the piston comprising: anannular body including a crown portion defining a longitudinal axis, aradial direction perpendicular to the longitudinal axis, a planecontaining the longitudinal axis and the radial direction, and acontoured combustion bowl; wherein in the plane containing thelongitudinal axis and the radial direction the crown portion includes atop squish surface, and a swirl pocket having a reentrant surface thatextends axially downwardly and radially outwardly from the top squishsurface defining a tangent that forms a reentrant angle with the topsquish surface; the swirl pocket further including: a concave arcuatesurface extending from the reentrant surface, a bottom concave arcuatesurface defining a bottom extremity of the swirl pocket, and a convexarcuate surface extending between the concave arcuate surface and thebottom concave arcuate surface; and a converging surface that extendsradially inwardly from the concave arcuate surface toward thelongitudinal axis, defining a lower tangent that forms an acute anglewith the longitudinal axis in the plane containing the longitudinal axisand the radial direction.
 12. The piston of claim 11 wherein thereentrant angle is larger than the acute angle.
 13. The piston of claim11 wherein the swirl pocket further includes a cylindrical surfaceextending downwardly from the convex arcuate surface to the bottomconcave arcuate surface.